1. Field of the Invention
This invention relates to a process using chemical vapor deposition for the manufacture of chemical vapor deposited structures, such as discs, domes, and plates. In particular this invention relates to an improved process for providing enhanced control of the rate of zinc vapor supplied to the chemical vapor deposition process.
2. Description of Related Art
In the manufacture of ZnS or ZnSe components by chemical vapor deposition (CVD), as shown in FIG. 1, a mold upon which the CVD structure is to be deposited is placed in a CVD furnace above a source of zinc metal. Typically, the mold is made from a number of graphite mandrel plates which are arranged in the form of a rectangular box and bolted together. The CVD furnace is then covered, sealed, and vacuum connections are attached in preparation for operation of the CVD process. The furnace is heated to operating temperature (650.degree.-700.degree. C.) and a flow of reaction gas, H.sub.2 S or H.sub.2 Se, and an inert carrier gas, such as argon, is initiated. The CVD process continues until a sufficient depth of material is deposited, after which the furnace is cooled (approximately 24 hours) and the mold containing the CVD deposited structures is disassembled and the structures are removed from the furnace.
The current method of supplying zinc vapor to the reaction, as shown in FIG. 1 consists of graphite retorts 10 located inside the CVD furnace 12 below the mandrel plates 14 upon which the CVD parts are deposited. The retorts are loaded with solid zinc, either in the form of balls or disks. The retorts 10 contain the supply of zinc for the entire run (135-455 kg. capacity). These retorts are then heated to approximately 650.degree. C. to melt and then evaporate the zinc. The evaporation rate of zinc is a function of the surface area of the molten zinc 16, temperature at the surface of molten zinc, and pressure in the retorts 10. The measurement of the zinc evaporation rate is done by calculating the change in volume of the molten zinc 16 over time. The change in volume is calculated by periodically measuring the level of the molten zinc 16 and calculating the rate by dividing the change in the level of the molten zinc by the period of time between which the readings were taken. The measurement of the liquid zinc level is done with a graphite float 18 on the zinc surface 20 which supports a graphite rod 22, a quartz connecting rod 24, and the magnetic core 26 of a linear variable differential transformer 28 (LVDT). As the zinc evaporates the level decreases and the core moves inside the LVDT to produce an electrical signal that is sent to an output device. This method has a number of disadvantages as discussed in the following paragraphs.
The temperature at which the molten zinc is to be maintained varies with each run due to the flow rate of zinc required, the condition of the insulation, the temperature of the cooling water, and the amount of zinc in the retort. The temperature is measured at the outer surface of the retort. The retort temperature at the beginning of the run is selected based on operating data from previous runs. Normally, the temperature of the molten zinc has to be adjusted during the course of the run due to changes which take place during the run that affect the surface area of the molten zinc and the pressure within the CVD furnace. For example, the surface area of the liquid zinc can be decreased by contamination, or "slag," that floats on the surface. Also, the retort pressure may also change due to contaminants collected by the zinc filter. The amount of zinc in the retort is constantly decreasing. These events may require a change in the retort temperature. Several data points must be collected to properly determine the necessary temperature adjustment of the molten zinc. Such adjustments usually take several hours.
Adjusting the temperature of the molten zinc is further complicated by a time lag between a change in temperature and when the effect is noticed due to the large mass of material present in the retort. Also, the first measurement of the molten zinc level after a change has been made may appear to be the opposite of the desired effect and may mask the actual evaporation rate. When the temperature is increased to increase the evaporation rate of the zinc, the zinc and graphite expand. A measurement of the zinc level at this time makes it appear as though the evaporation rate is decreasing, and vice versa. It may take several hours of process time to obtain the correct measurement.
In addition, the reading from the LVDT is difficult to evaluate due to the graphite float on the unstable surface of the liquid zinc. The float and connecting rods also tend to stick and drag on side wall of the LVDT protection tube and openings. Again, due to these difficulties is may take several hours of process time to determine the true trend of the evaporation rate.
Further, the assembly of the retorts and the LVDT is difficult. The assembly requires filling multiple retorts with large amounts of zinc, and the alignment of the LVDT. The success rate of this operation is approximately 80%. In addition, the length of the runs is limited by the amount of zinc that can be loaded into the retorts. Also, a large amount of energy is consumed due to the large mass of the zinc, retort, and furnace which must be heated so that the zinc metal can be evaporated.
During the CVD process, it is desirable to provide a constant supply of zinc vapor to be reacted with a reaction gas in the presence of mandrel plates to form a CVD structure. Further, it is desirable to provide instantaneous information on the rate of zinc vaporization during the CVD process.
A solution to the problem of providing metal vapor to a metal coating process is suggested by Strong, U.S. Pat. No. 3,086,889. Strong discloses a wire metal feed to a tapered insulated boat for providing metal vapor for applying a thin metal film directly to a dielectric sheet without a chemical reaction, under low pressure in a continuous operation.